World War II in 1945. These two events represent the only use of nuclear

weapons in war to date.

Following a firebombing campaign that destroyed many Japanese

cities, the Allies prepared for a costly invasion of Japan. The war in

Europe ended when Nazi Germany signed its instrument of surrender on

8 May, but the Pacific War continued. Together with the United Kingdom

and the Republic of China, the United States called for a surrender of Japan in the Potsdam Declaration on 26 July 1945, threatening Japan with

"prompt and utter destruction". The Japanese government ignored this

ultimatum, and the United States deployed two nuclear weapons developed by the Manhattan Project. American airmen dropped Little

Boy on the city of Hiroshima on 6 August 1945, followed by Fat Man

over Nagasaki on 9 August.

Within the first two to four months of the bombings, the acute effects killed 90,000–166,000 people in Hiroshima and 60,000–80,000 in

Nagasaki, with roughly half of the deaths in each city occurring on the first day. The Hiroshima prefecture health department estimated that, of the people who died on the day of the explosion, 60% died from flash or flame burns, 30% from falling debris and 10% from other causes. During the following months, large numbers died from the effect of burns, radiation sickness, and other injuries, compounded by illness. In a US

estimate of the total immediate and short term cause of death, 15–20%

died from radiation sickness, 20–30% from burns, and 50–60% from

other injuries, compounded by illness. In both cities, most of the dead were civilians, although Hiroshima had a sizeable garrison.

On 15 August, six days after the bombing of Nagasaki, Japan announced its surrender to the Allies, signing the Instrument of Surrender on 2

September, officially ending World War II. The bombings led, in part, to post-war Japan's adopting Three Non-Nuclear Principles, forbidding the

nation from nuclear armament. The role of the bombings in Japan's surrender and their ethical justification are still debated.

Pacific War

Background

Main article: Pacific War

In 1945, the Pacific War between the Empire of Japan and the Allies of

World War II had entered its fourth year. World War II was not winding

down. Instead, the fighting was being prosecuted with ever-increasing fury. Of the 1.25 million battle casualties incurred by the United States in World War II, nearly one million occurred in the twelve month period from June 1944 to June 1945. December 1944 saw American battle casualties hit an all-time monthly high of 88,000 as a result of the German Ardennes [2]Offensive.

In the Pacific during this period, the Allies captured the Mariana and [3][4][5]Palau Islands,returned to the Philippines, and invaded Borneo. The

policy of bypassing Japanese forces was abandoned. In order to free troops for use elsewhere, offensives were undertaken to reduce the Japanese [6]forces remaining in Bougainville, New Guinea and the Philippines. In

April 1945, American forces had landed on Okinawa, where heavy fighting

would continue until June. Along the way, the ratio of Japanese deaths

to American casualties dropped from 5 to 1 in the Philippines to 2 to 1 [2]on Okinawa.

Preparations to invade Japan

Anti-Japanese poster depicting the Bataan Death March thereby emphasising earlier defeats rather than later victories in the Pacific

Operation DownfallMain article:

Even before the surrender of Nazi Germany on 8 May 1945, plans were already underway for the largest operation of the Pacific War, Operation Downfall, [7]the invasion of Japan. The operation had two parts: Operations Olympic and Coronet. Set to begin in October 1945, Olympic involved a series of landings by the US Sixth Army intended to capture the southern third of [8]the southernmost main Japanese island, Kyūshū. Operation Olympic was

to be followed in March 1946 by Operation Coronet, the capture of the Kantō

Plain, near Tokyo on the Japanese island of Honshū by the US First, Eighth

and Tenth Armies. The target date was chosen to allow for Olympic to complete its objectives, troops to be redeployed from Europe, and the [9]Japanese winter to pass.

Japan's geography made this invasion plan obvious to the Japanese as well; they were able to predict the Allied invasion plans accurately and thus adjust their defensive plan, Operation Ketsugō, accordingly. The Japanese planned an all-out defense of Kyūshū, with little left in reserve for any [10]subsequent defense operations. Four veteran divisions were withdrawn from the Kwantung Army in Manchuria in March 1945 to strengthen the forces [11]in Japan, and 45 new divisions were activated between February and May 1945. Most were immobile formations for coastal defence, but 16 were high [12]quality mobile divisions. In all, there were 2.3 million Japanese Army troops prepared to defend the Japanese home islands, another 4 million Army and Navy employees, and a civilian militia of 28 million men and women.

Casualty predictions varied widely, but were extremely high. The Vice Chief of the Imperial Japanese Navy General Staff, Vice AdmiralTakijirō [13]Ōnishi, predicted up to 20 million Japanese deaths.

US Army poster prepares the public for the invasion of Japan after ending war on Germany

[14]A study from 15 June 1945 by the Joint War Plans Committee, who provided

planning information to the Joint Chiefs of Staff, estimated that Olympic

would result in between 130,000 and 220,000 US-casualties of which U.S. dead would be the range from 25,000 to 46,000. Delivered on 15 June 1945 after insight gained from the Battle of Okinawa, the study noted Japan's inadequate defenses due to the very effective sea blockade and the American firebombing campaign. The Chief of Staff of the United States

Army, General of the ArmyGeorge C. Marshall and General of the Army

Douglas MacArthur signed documents agreeing with the Joint War Plans [15]Committee estimate.

The Americans were alarmed by the Japanese build up, which was accurately [16]tracked through Ultra intelligence.United States Secretary of War

Henry Lewis Stimson was sufficiently concerned about high American estimates of probable casualties to commission his own study by Quincy

Wright and William Shockley. Wright and Shockley spoke with Colonels James

McCormack and Dean Rusk, and examined casualty forecasts by Michael

DeBakey and Gilbert Beebe. Wright and Shockley estimated the invading Allies would suffer between 1.7 and 4 million casualties in such a scenario, of whom between 400,000 and 800,000 would be dead, while [17][18]Japanese casualties would have been around 5 to 10 million.

Marshall began contemplating the use of a weapon which was "readily [19]available and which assuredly can decrease the cost in American lives":

chloride were moved to Luzon from stockpiles in Australia and New Guinea

in preparation for Operation Olympic, and General of the Army Douglas

MacArthur ensured that Chemical Warfare Service units were trained in [19]their use.

Air raids on Japan

Main article: Air raids on Japan

A B-29 over Osaka on 1 June 1945

While the United States had developed plans for an air campaign against Japan prior to the Pacific War, the capture of Allied bases in the western Pacific in the first weeks of the conflict meant that this offensive did not begin until mid-1944 when the long-ranged Boeing B-29 Superfortress

became ready for use in combat. Operation Matterhorn involved India-based

B-29s staging through bases around Chengtu in China to make a series of

raids on strategic targets in Japan between June 1944 and January 1945. This effort proved unsuccessful due to logistical difficulties with the remote location, technical problems with the new and advanced aircraft, [20]unfavourable weather conditions, and ultimately enemy action.

USAAFBrigadier GeneralHaywood S. Hansell determined that Guam, Tinian

and Saipan in the Mariana Islands would better serve as B-29 bases, but

they were in Japanese hands. Strategies were shifted to accommodate the air war, and the islands were captured between June and August 1944. Air

bases were developed, and B-29 operations commenced from the Marianas in November 1944, greatly expanding the scope of the strategic bombing [20]campaign against Japan.

These attacks initially targeted key industrial facilities, but from March 1945 they were frequently directed against urban areas. The capture of Okinawa in June 1945 provided airfields even closer to the Japanese mainland, allowing the bombing campaign to be escalated further. Over the next six months, the XXI Bomber Command fire-bombed 67 Japanese cities.

The Operation Meetinghouse 9–10 March Bombing of Tokyo caused

280,000–100,000 casualties and destroyed 16 square miles (41 km) of the

city with 267,000 buildings–the deadliest of the war. Aircraft flying from Allied aircraft carriers and the Ryukyu Islands also regularly struck [21]targets in Japan during 1945 in preparation for Operation Downfall.

The Japanese military was unable to stop the Allied attacks, and the country's civil defense preparations proved inadequate. From April 1945, the Japanese Army and Naval Air Forces stopped attempting to intercept the air raids in order to preserve fighter aircraft to counter the expected [22]invasion. By mid-1945 the Japanese also only occasionally scrambled aircraft to intercept individual B-29s conducting reconnaissance sorties [23]over the country in order to conserve supplies of fuel. By July 1945,

the Japanese had stockpiled 1,156,000 US barrels (137,800,000 l; 36,400,000 US gal; 30,300,000 imp gal) of avgas for the invasion of [24]Japan.

Atomic bomb development

Main article: Manhattan Project

Working in collaboration with the United Kingdom and Canada, with their [25][26]respective projects Tube Alloys and Chalk River Laboratories, the

Manhattan Project, under the direction of Major GeneralLeslie Groves,

of the U.S. Army Corps of Engineers, designed and built the first atomic [27]bombs. Preliminary research began in 1939, originally in fear that the [28]Nazi atomic bomb project would develop atomic weapons first. In May 1945, [29]the defeat of Germany caused the focus to turn to use against Japan.

Two types of bombs were eventually devised by scientists and technicians at Los Alamos under American physicistRobert Oppenheimer. The Hiroshima

bomb, known as Little Boy, was a gun-type fission weapon made with

uranium-235, a rare isotope of uranium extracted in giant factories in [30]Oak Ridge, Tennessee. The other was an implosion-type nuclear weapon

using plutonium-239, a synthetic element created in nuclear reactors at

Hanford, Washington. A test implosion weapon, the gadget, was detonated [31]at Trinity Site, on 16 July 1945, near Alamogordo, New Mexico. The [32]Nagasaki bomb, Fat Man was also an implosion device.

Preparations

Organization and training

Aircraft of the 509th Composite Group that took part in the Hiroshima bombing. Left to right: backup plane, The Great Artiste, Enola Gay

The 509th Composite Group was constituted on 9 December 1944, and

activated on 17 December 1944, at Wendover Army Air Field, Utah, commanded [33]by ColonelPaul W. Tibbets. Tibbets was assigned to organize and command a combat group to develop the means of delivering an atomic weapon against targets in Germany and Japan. Because the flying squadrons of the group consisted of both bomber and transport aircraft, the group was designated as a "composite" rather than a "bombardment" unit.

Working with the Manhattan Project at Site Y in Los Alamos, New Mexico,

Tibbets selected Wendover for his training base over Great Bend, Kansas, [34]and Mountain Home, Idaho because of its remoteness. On 10 September 1944,

from the 504th Bombardment Group (Very Heavy) at Fairmont Army Air Base,

Nebraska, where it had been in group training since 12 March. When its parent group deployed to the Mariana Islands in early November 1944, the

squadron was assigned directly to the Second Air Force until creation of [35]the 509th Composite Group. Originally consisting of twenty-one crews,

fifteen were selected to continue training and were organized into three flights of five crews, lettered A, B, and C.

The 320th Troop Carrier Squadron, the other flying unit of the 509th, came into being because of the highly secret work of the group. The organization that was to become the 509th required its own transports for the movement of both personnel and materiel, resulting in creation of an ad hoc unit [35][36]nicknamed "The Green Hornet Line". Crews for this unit were acquired

from the six 393rd crews not selected to continue B-29 training, some of whom chose to remain with the 509th rather than be assigned to a

replacement pool of the Second Air Force. They began using Curtiss C-46

Commandos and C-47 Skytrains already at Wendover, and after November 1944 [37]flew five acquired C-54 Skymasters. The 320th Troop Carrier was formally [33]activated at the same time as the group.

Other support units were activated at Wendover from personnel already present and working with its Project W-47, which was later superseded by Project Alberta, or in the 216th Base Unit, both of which were affiliated with the Project Y. The 390th Air Service Group was created as the command echelon for the 603rd Air Engineering Squadron, the 1027th Air Material squadron, and its own Air Base Support Squadron, but as these units became independent operationally, acted as the basic support unit for the entire 509th Composite Group in providing quarters, rations, medical care, postal service and other basic support functions. The 603rd Air Engineering Squadron was unique in that it provided depot-level B-29 maintenance in the field, obviating the necessity of sending aircraft back to the United States for major repairs. The 603rd made a number of modifications to the first contract order of Silverplate B-29s that were

later incorporated as specifications for the combat models. The 393rd Bomb Squadron began replacement of its original B-29s with modified Silverplate aircraft with the delivery of three new B-29s in [36]mid-October 1944. These aircraft had extensive bomb bay modifications and a "weaponeer" station installed, but initial training operations identified numerous other modifications necessary to the mission, particularly in reducing the overall weight of the aircraft to offset the heavy loads it would be required to carry. Five more Silverplates were delivered in November and six in December, giving the group 14 for its training operations. In January and February 1945, 10 of the 15 crews under the command of the Group S-3 (operations officer) were assigned temporary duty at Batista Field, San Antonio de los Baños, Cuba, where they trained [38]in long-range over-water navigation.

On 6 March 1945, the 1st Ordnance Squadron (Special, Aviation) was activated at Wendover, again from Army Air Forces personnel on hand or already at Los Alamos, and concurrent with the activation of Project

Alberta. Its purpose was to provide trained personnel and special equipment to the group to enable it to assemble atomic weapons at its operating base, thereby allowing the weapons to be transported more safely in their component parts. A rigorous candidate selection process was used to recruit personnel, with reportedly an 80% "washout" rate, and those made a part of the unit were not permitted transfer until the end of the war, nor were they allowed to travel without escorts from Military [39]Intelligence units.

With the addition of the 1st Ordnance Squadron to its roster, the 509th Composite Group had an authorized strength of 225 officers and 1,542 enlisted men, almost all of whom deployed to Tinian. The 320th Troop Carrier Squadron did not officially deploy but kept its base of operations at Wendover. In addition to its authorized strength, the 509th had attached to it on Tinian 51 civilian and military personnel of Project [40][41]Alberta, known as the 1st Technical Detachment. There were two

the deputy commander of the Manhattan Project, and Rear AdmiralWilliam [42]R. Purnell of the Military Policy Committee. They were on hand to decide

higher policy matters on the spot. Along with CaptainWilliam S. Parsons,

the commander of Project Alberta, they became known as the "Tinian Joint [43]Chiefs".

The 509th began replacement of its 14 training Silverplates in February 1945 by transferring four to the 216th Base Unit. In April they began receiving Silverplates of the third modification increment and the remaining ten training B-29s were placed in storage. Each bombardier completed at least 50 practice drops of inert pumpkin bombs and Tibbets [44]declared his group combat-ready. Preparation for Overseas Movement (POM)